Figure 2.

Age effects on DNA methylation levels are well preserved between blood and brain tissue. (a-d) Scatterplots of mean CpG methylation levels in whole blood of the Dutch samples (x-axis)
and corresponding mean brain methylation values (y-axis) for frontal cortex (FCTX)
(a), temporal cortex (TCTX) (b), pons (c), and cerebellum (CRBLM) (d). The brain methylation
data used were obtained from [19]. Note that strong correlations (around r = 0.9) exist between the mean methylation
levels in whole blood and brain tissue. We hypothesize that the relatively low correlation
of r = 0.85 for cerebellum may reflect DNA quality. (e-g) Age correlations of CpG methylation levels show moderate preservation (correlations
around 0.33) between blood (x-axis) and brain tissues (y axes). Analogous to Figure
1, the methylation levels of each gene (represented by a dot) were correlated with
subject age and a linear regression model was used to calculate a correlation test
P-value. The x-axis of each scatterplot shows the (signed) logarithm (base 10) of the
correlation test P-value in blood. Genes with a significant positive (negative) correlation with age
have a high positive (negative) log P-value. The y-axis shows the corresponding correlation test P-values in the frontal cortex (e), temporal cortex (f), pons (g), and cerebellum (h).